This invention relates to an antenna which is installed on a flat surface (or a slightly curved surface), and more particularly, to a circularly polarized antenna which is installed on a window glass for a vehicle and is suitable to receive satellite broadcasting signal.
Conventionally, as a planar antenna for satellite communication such as GPS satellite communication, XM satellite broadcasting, or Sirius satellite broadcasting, a microstrip antenna is widely known. The microstrip antenna is installed on a wide metal plate such as a metal roof of a vehicle, to thereby yield high reception performance both for radio waves arriving at a high elevation angle and for radio waves arriving at a low elevation angle.
Such a conventional microstrip antenna needs to be installed on a metal surface, and therefore has a difficulty in being installed on a window glass of a vehicle. However, mainly in view of vehicle designing, there is currently a need for an antenna for receiving satellite broadcasting waves which is installed on a window glass of a vehicle instead of on a body thereof. As such an antenna that is installed on a window glass of a vehicle instead of on a metal surface thereof, antennas disclosed in WO 2003/105278 A1, JP 2004-214820 A, JP 2004-214819 A, and JP 2008-141765 A are known.
The antennas installed on a window glass of a vehicle which are disclosed in WO 2003/105278A1, JP 2004-214820 A, and JP 2004-214819 A are coplanar antennas which may be formed only on one surface of a dielectric substrate. Such an antenna as described above requires a wide conductive area, and hence has a problem that visibility decreases when the antenna is installed on a window glass. The antenna described above has another problem that performance thereof decreases when the antenna is installed on a vehicle.
The antenna disclosed in JP 2008-141765 A is a microstrip antenna which is intended to be installed on a window glass, and has a problem that visibility of a view through the window glass decreases as in the cases of WO 2003/105278A1, JP 2004-214820 A, and JP 2004-214819 A.